Objective: To elucidate the mechanisms regulating adenovirus replication and the viral gene function(s) essential for cell transformation. Approach: Conditionally lethal, temperature-sensitive mutants of type 5 and 12 adenoviruses will be employed. Particular emphasis will be placed upon studies using two unique DNA minus mutants to study mechanisms of DNA replication and transcription as well as transformation. One of these mutants, H5ts125, transforms 4 to 8 times more rat cells than either wild-type virus or the other DNA minus mutant, H5ts149. We will test a model which predicts that in ts 125-transformed cells a larger proportion of the viral genome is integrated, and that the defective viral gene product, a single-stranded DNA-binding protein, plays a modulating role in adenovirus transformation.